HgTe colloidal quantum dots (CQDs) are promising absorber systems for infrared detection due to their widely tunable photoresponse in all infrared regions. Up to now, the best-performing HgTe CQD photodetectors have relied on using aggregated CQDs, limiting the device design, uniformity and performance. Herein, we report a ligand-engineered approach that produces well-separated HgTe CQDs. The present strategy first employs strong-binding alkyl thioalcohol ligands to enable the synthesis of well-dispersed HgTe cores, followed by a second growth process and a final postligand modification step enhancing their colloidal stability. We demonstrate highly monodisperse HgTe CQDs in a wide size range, from 4.2 to 15.0 nm with sharp excitonic absorption fully covering short- and midwave infrared regions, together with a record electron mobility of up to 18.4 cm2 V–1 s–1. The photodetectors show a room-temperature detectivity of 3.9 × 1011 jones at a 1.7 μm cutoff absorption edge.
Colloidal quantum dots (CQDs) are the category of semiconductor nanocrystals with sizes smaller than their exciton Bohr radius. [1][2][3][4][5] Given the nanoscale size, CQDs exhibit strong quantum confinement effect, which induce many unique optical properties such as tunable absorption, with exciton peaks ranging from the ultraviolet to the infrared, to the THz region. [6][7][8][9][10] This size-dependent absorption makes CQDs quite suitable for single-junction and tandem solar energy harvesting. [11][12][13][14][15][16][17][18][19][20][21][22][23][24] Among various synthesis approaches of CQDs, Solution-processed colloidal quantum dots (CQDs) are promising candidates for the third-generation photovoltaics due to their low cost and spectral tunability. The development of CQD solar cells mainly relies on high-quality CQD ink, smooth and dense film, and charge-extraction-favored device architectures. In particular, advances in the processing of CQDs are essential for high-quality QD solids. The phase transfer exchange (PTE), in contrast with traditional solid-state ligand exchange, has demonstrated to be the most promising approach for high-quality QD solids in terms of charge transport and defect passivation. As a result, the efficiencies of Pb chalcogenide CQD solar cells have been rapidly improved to 14.0%. In this review, the development of the PTE method is briefly reviewed for lead chalcogenide CQD ink preparation, film assembly, and device construction. Particularly, the key roles of lead halides and additional additives are emphasized for defect passivation and charge transport improvement. In the end, several potential directions for future research are proposed.
Lead selenide (PbSe) colloidal quantum dots (CQDs) are promising candidates for optoelectronic applications. To date, PbSe CQDs capped by halide ligands exhibit improved stability and solar cells using these CQDs as active layers have reported a remarkable power conversion efficiency (PCE) up to 10%. However, PbSe CQDs are more prone to oxidation, requiring delicate control over their processability and compromising their applications. Herein, an efficient strategy that addresses this issue by an in situ cation‐exchange process is reported. This is achieved by a two‐phase ligand exchange process where PbI2 serves as both a passivating ligand and cation‐source inducing transformation of CdSe to PbSe. The defect density and carrier lifetime of PbSe CQD films are improved to 1.05 × 1016 cm−3 and 12.2 ns, whereas the traditional PbSe CQD films possess 1.9 × 1016 cm−3 defect density and 10.2 ns carrier lifetime. These improvements are translated into an enhancement of photovoltaic performance of PbSe solar cells, with a PCE of up to 11.6%, ≈10% higher than the previous record. Notably, the approach enables greatly improved stability and a two‐month stability is successfully demonstrated. This strategy is expected to promote the fast development of PbSe CQD applications in low‐cost and high‐performance optoelectronic devices.
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